Revisiting huntingtin activity and localization signals in the context of protein structure.

Protein localization signals and activity motifs have been defined within huntingtin since 2003. Advances in technology in protein structure determination by cryo-electron microscopy (EM) have led to 2.6 Å resolution structures of huntingtin and HAP40 for the majority of the protein, although structure of the amino terminus with the polyglutamine expansion remains elusive in the context of full-length huntingtin. Recent advances in protein modeling using neural network algorithms trained on a database of known protein structures has resulted in structure predictions that are useful for researchers but need experimental validation. Here, we use both structures solved by cryo-EM as well as modeling centered around experimental structural data to retrospectively revisit huntingtin protein localization signals identified prior to the cryo-EM and AI-enabled structural revolutions. We interrogate these models as well as put forward testable hypotheses of allosteric changes in huntingtin and how they could be affected by polyglutamine expansion. We also extended this methodology to another polyglutamine disease protein, ataxin-1, expanded in Spinocerebellar Ataxia Type 1 (SCA1).